1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high-powered pulsed lasers, in the shock processing of solid materials, and more particularly, a method for improving properties of solid materials by providing shock waves therein. The invention is especially useful for enhancing or creating desired physical properties such as hardness, strength, and fatigue strength.
2. Description of the Related Art
Older methods for the shock processing of solid materials typically involve the use of high-explosive materials in contact with the solid, or high-explosive materials or high-pressure gases to accelerate a plate that strikes a solid to produce shock waves therein. Such methods have several disadvantages. For example: (a) it is difficult and costly to shock process non-planar surfaces and complicated geometries, (b) storage and handling of the high-explosive materials and high-pressure gases pose a hazard, (c) the processes are difficult to automate and thus, fail to meet some industrial needs, and (d) high-explosive materials and high-pressure gases cannot be used in extreme environments such as high temperatures and high vacuum.
Shot peening is another widely known and accepted process for improving the fatigue, hardness, and corrosion resistance properties of materials by impact treatment of their surfaces. In shot peening, many small shots or beads are thrown at high-speed against the surface of a material. The shot or beads sometime escape from the treatment equipment and scatter in the surrounding area. Since the shot or beads might get into surrounding machinery and cause damage, shot peening usually cannot be used in a manufacturing line. Ordinarily, shot peening cannot be used on machined surfaces without a likelihood of damaging them. In addition, shot peening has problems maintaining consistency of treatment caused by inherent wear of the shot and the shot peening equipment.
Laser shock processing equipment, however, can be incorporated into manufacturing lines without damage to the surrounding equipment. Shock processing with coherent radiation has several advantages over what has been done previously. For example, the source of the radiation is highly controllable and reproducible. The radiation is easily focused on pre-selected surface areas and the operating mode is easily changed. This allows flexibility in the desired shocking pressure and careful control over the workpiece area to be shocked. Workpieces immersed in hostile environments, such as high temperature and high vacuum can be shock processed. Additionally, it is easy to shock the workpiece repetitively. This is desirable where it is possible to enhance material properties in a step-wise fashion.
Laser peening (here and after referred to as laser shock processing) utilizes two overlays: a transparent overlay (e.g. water) and an opaque layer, (e.g. an oil based or acrylic-based black paint). Processing is typically done with the workpiece at ambient or room temperature. During processing, a laser beam is directed to pass through the transparent overlay and is absorbed by the opaque layer, e.g. black paint, causing a rapid vaporization of the paint surface and the generation of a high amplitude shock wave. The shock wave cold-works the surface of the part and creates compressive residual stresses which provide an increase in fatigue properties of the part. A workpiece is typically processed by processing a matrix of overlapping spots that cover the fatigue critical zone of the part.
Solid materials subject to laser shock processing contain naturally occurring dislocations. These dislocations move through the matrix of the solid material when the solid material is subject to stresses such as bending or pounding. Laser shock processing introduces additional dislocations in the solid material which increase material strength and contribute to residual stress.
One problem with current methods of laser shock processing is that some solid materials, at room temperature, are too brittle to process. When laser processing of workpieces of these materials is done at ambient or room temperature, these material will crack or fracture. An example of a class of solid materials which are brittle at room temperatures, but whose ductility slowly increases with increasing temperatures, are many inter-metallic compounds. Therefore, these materials, as well as others, which may benefit from laser shock processing are prevented from being processed, because of their tendency to crack and break.
An additional problem with current methods of laser shock processing is the inability to modify the amount of compressive residual stresses previously introduced in a solid material by laser shock processing. Once the compressive residual stress is introduced in a solid material, the magnitude or amount of compressive residual stress cannot be altered via the current laser shock processing methods, particularly to reduce the magnitude, if desired.